The invention will now be described and discussed in terms of one type of repetitive signal, a video signal. However, this should not be considered limiting and is for ease of understanding only. It is to be understood that the invention is applicable to any signal which includes a repetitive synchronisation signal.
Synchronisation signal separators (sync. separators) are widely used in many types of video equipment, for example in video synchronisers which introduce a delay into the incoming video signal in order to synchronise it with other video sources. Video synchronisers perform well with good quality video signals, but their performance is less successful with noisy video such as may occur when processing signals from radio-cameras. Examples of such radio-cameras are described in BBC Research Department Report 1991/15 entitled `The Development of Radio Systems for Portable Cameras Used in Television Production` by C. Gandy. Television signals received from these mobile cameras using microwave links are often of variable quality due to noise and signal loss. This causes problems for video synchronisers, in particular their sync. separator circuitry. Existing sync. separators have been found to be inadequate, as they are prone to losing the lock on the signal in conditions of severe noise, rather than `flywheeling`, a condition in which the sync. separator would generate its own sync. signals without reference to the incoming syncs. Moreover, in the known apparatus, the frequency of the flywheel decays as the control voltage held on a capacitor gradually discharges. Thus, the timing of the sync. signals generated during flywheeling becomes progressively more inaccurate.
A further disadvantage of existing sync. separators is that they can only operate with standard level video signals such as are generated under studio conditions. The noisy environments in which radio cameras operate mean that standard level signals are often not produced.